This invention relates to a carrier which is used as a workpiece holder in combination with a workpiece polishing apparatus. More particularly, this invention relates to flat planar carriers, made of polymeric materials, which retain or hold items for polishing metal, semiconductor materials or other substrates by appropriately positioning those materials or substrates in a polishing machine and maintaining the materials or substrates in the desired position in such a machine.
The metal or semiconductor materials which are to be polished are preferably in the form of wafers or discs which require extremely flat, polished surfaces to provide maximum useful surface area so they may be fabricated as computer or electronic components. Also, there are other fields which require wafers of metals or semiconductor materials wherein one or both surfaces of each wafer must be appropriately polished to a high degree of flatness with minimum surface distortion and imperfection.
Apparatus have been developed for polishing of wafers of this type, including wafer polishing apparatus as disclosed in U.S. Pat. No. 5,329,732 issued Jul. 19, 1994 to Karlsrud et al. for a Wafer Polishing Method and Apparatus, which patent is incorporated herewith by reference. Another apparatus of this general type used for such purposes is disclosed in U.S. Pat. No. 5,274,960 issued Jan. 4, 1994 in the name of Karlsrud for a Uniform Velocity Double Sided Finishing Machine, which patent is also incorporated herewith by reference.
In polishing apparatus of the type disclosed in the reference patents, carrier sheets or carrier gears are used to retain and position the wafers that are being polished between polishing platens. Thus, wafers, which have a nominal thickness, are retained in openings defined in flat carrier sheets. The polishing apparatus will then effect a polishing action on the surface of the wafer materials as they are retained and maintained in position in the carrier sheet. The carrier sheets or carriers may have various shapes, and the openings for retention of the wafers in the sheets may also have various shapes and/or configurations, generally congruent with shape of the wafer held by the sheet. Further, the carriers may have an outside, peripheral gear coactive with a driving gear of the polishing apparatus to thereby facilitate movement of the carriers and the wafers retained thereby during the polishing operation.
In the past, carriers were usually made from metals such as sheet steel, aluminum or thermoplastic polymers having moderate mechanical properties. Polymers such as polycarbonates, acrylics or fiber reinforced thermosetting resin such as glass fiber/epoxy, carbon fiber/epoxy or natural fiber (cotton or linen)/phenolic were used as carrier materials. A disadvantage observed in the use of metal carriers is that metal particles will abrade from the carrier or carrier gear, usually from the gear teeth of the carrier or from the surface. The particles may then cause scratching of the wafers which are being polished as they are held by the carrier.
A difficulty observed with prior art carriers made from thermoplastic polymers having moderate mechanical properties is that such polymers do not retain their shape, particularly if the carrier (carrier gear or carrier sheet) is very thin i.e. less than a millimeter, for example. This problem is exacerbated if the carrier is of a large diameter, for example, about 250 millimeters to 750 millimeters. Such polymers are also not sufficiently stiff to remain flat as a thin sheet or film and therefore may result in damage to the object or wafer which is being polished when held by such a carrier. Of course the objects being polished are typically rather thin. Thus, it is necessary to provide a thin, uniform thickness carrier having a thickness which is slightly less than the finished thickness of the object being polished.
A difficulty observed with carrier gears or carriers made from fiber reinforced thermosetting resins is that they are often brittle. Also, the reinforcement materials may be harder than the material being polished, for example if the reinforcement materials are glass or carbon fibers. In such circumstances, the reinforcement materials may abrade the wafer which is to be polished. That is, pieces of the hard materials, such as the carbon fiber, will come into contact with the wafer surface being polished and scratch that surface, causing an unsatisfactory finish.
In sum, it is a desirable and beneficial objective that such carriers be designed to retain a flat object such as a disc or wafer in a fixed position during grinding and/or polishing. Such carriers may or may not be in the form of a spur gear. Such carriers must have a high degree of strength and flexural modulus so as to appropriately hold, retain and maintain the object being polished. The carrier or carrier gear should also be comprised of materials which will not flake, shred or shard or otherwise interfere with the polishing activity or action. These objectives, among others, inspired the development of carrier gear construction disclosed and claimed.